1. Field of the Invention
The present invention relates to a round window driving transducer for easy implantation and an implantable hearing device having the same. More particularly, the round window driving transducer of the present invention is designed to be easily implanted in the round window of the cochlea in the middle ear cavity, and has excellent high frequency characteristics, which can effectively assist patients with sensorineural hearing loss to hear sound better.
2. Description of the Related Art
In general, about 10% of the world population has various types of hearing loss. About 8% of those with hearing loss are deaf people whose auditory threshold is too high, whereas the remainder is suffering from mild, moderate or severe hearing loss.
In order to compensate for the moderate hearing loss and the severe hearing loss with the auditory threshold ranging from 55 dB HL to 90 dB HL, various designs of implantable middle ear hearing devices, which can be totally implanted without being exposed, have been proposed.
For example, various implantable middle ear hearing devices have been disclosed in U.S. Pat. Nos. 5,800,336, 5,558,618, 6,277,148, 5,360,338, 5,277,694, 5,772,575, 5,951,601 and so on. In such implantable middle ear hearing devices, a vibrator is regarded as the most important part. In a fully-implantable middle ear hearing device, the vibrator is required not only to be highly efficient but also to be small and adequate for implantation so as to be easily inserted into the middle ear.
A floating mass transducer (U.S. Pat. No. 5,800,336) of Ball et al. is currently commercialized. In spite of many advantages, the above identified transducer has a drawback in that low and high frequency bands except for a middle frequency band have a low gain. Further, this transducer cannot be used in a Magnetic Resonance Imaging (MRI) system that has a strong magnetic field.
Since the transducer is suspended from the ossicle by means of a clip, a portion of the ossicle in contact with the transducer could be weakened by the load of the transducer suspending therefrom, thereby loosening the coupling between the transducer and the ossicle.
It is expected that several types of transducers for implantable middle ear hearing devices available from Otologics, Implex AG and St. Croix have drawbacks such as a difficult implant surgery and prolonged surgery time. This is because a transducer has to be fixed at one end thereof to the wall of the middle ear cavity before the other end thereof is brought into contact with the ossicle in order to be implanted inside the middle ear cavity, and then precision adjustment has also to be performed using small screws.
According to U.S. Pat. No. 5,772,575 to Lesinski et al. and U.S. Pat. No. 5,498,226 to Lenkauskas, a screw housing, with a piezoelectric vibrator disposed therein, is pushed into the inner ear by a drilling operation. However, these approaches are disadvantageous because of the leakage of perilymph and infection in the surgery.
US Patent Application Publication No. 2005/0020873A1 by Berrang et al. discloses a transducer having a construction similar to that of the floating mass transducer of Ball as described above. The transducer of Berrang et al. is constructed to vibrate a bone portion adjacent to the three semicircular canals, and includes a multilayer piezoelectric element with at least one vibration-reflecting mass at one end thereof and a transducer housing defining an enclosure.
The transducer of Berrang et al. having a diameter ranging from 2 mm to 6 mm and a length ranging from 2 mm to 5 mm is larger than the transducer of Ball et al., which has a diameter 1.8 mm and a length 2 mm. This is because the transducer of Berrang et al. is not directly mounted inside the middle ear or to the cochlea but is implanted in a bone portion between the superior and lateral semicircular canals by drilling through the temporal bone.
The vibrator used in the transducer of Berrang et al. needs a simpler surgery than the vibrator used in the floating mass transducer of Ball et al. However, since the vibrator transmits vibration signals through the bone without direct contact with the entrance of the inner ear or the cochlea, the vibration signals are attenuated while vibration energy is being transmitted to the entrance of the inner ear through the three semicircular canals and the vestibular organ.
In consequence, when the transducer of Berrang et al. is employed in a fully-implantable middle ear hearing device, it consumes much more power than the transducer of Ball et al. Further, since grooves formed in the outer surface of the transducer in order to improve osseointegration, the transducer has to directly contact the bone when it is vibrating in the bone. This, however, may degrade vibration performance. Consequently, in terms of efficiency, this type of transducer is substantially improper for the fully-implantable middle ear hearing device since it consumes a large amount of battery power.
Typically, sound is transmitted in the order of the outer ear, the tympanic membrane, the ossicle, the oval window of the cochlea, the endolymph and the round window. Considering the operating principle of the cochlea, sound entering the round window prior to the oval window can also be recognized by the vibration of the basilar membrane inside the cochlea. That is, many scholars have proved that the sound can be properly recognized even if they are transmitted in the order of the round window, the endolymph, the oval window and the ossicle. U.S. Pat. No. 5,360,388, invented by Spindel et al. of James Madison University, discloses a small electromagnetic transducer, which is attached to the round window and is driven by electric signals from an electromagnetic coil. The problems of the electromagnetic transducer are that it consumes too much power but also is not compatible with the MRI system.
Recently, V. Colletti (Italian scholar) et al. proposed a round window driving system in which a floating mass transducer is wrapped in a biocompatible tissue such as a soft fascia so as to be fixed in contact with the round window membrane. Here, the vibrator is constructed to freely vibrate the round window membrane in response to external electric signals.
This feature is significantly distinct from the transducer of Berrang et al., which is constructed to vibrate the bone, as is disclosed in US Patent Application Publication No. 2005/0020873A1. The round window niche is carefully drilled with a drill of 3 mm to 5 mm, and then the round window membrane is exposed so that it can be seen with the eye. Next, the round window membrane is covered with a thin piece of fascia, on which a vibrating cylinder is then placed, and finally, the cylinder is fixed by wrapping the entire part thereof with a fascia. It is reported according to clinical test results that this method greatly improved hearing ability.
However, according to V. Colletti's method as described above, a surgery has to be performed on a wide area since a large amount of the entrance of the round window of the cochlea is drilled so that the round window membrane can be seen with the eye when the floating mass transducer is implanted in the round window. Since the floating mass transducer cannot be fixed without being wrapped in the fascia, there is a risk that the cylinder type transducer may be moved out of the round window by external impact or shaking.
Furthermore, the round window membrane may be damaged when it is being drilled, and the process of wrapping the transducer in the fascia to prevent loosening or separation also requires surgeons to have high level of concentration that is burdensome.